This hasn't necessarily been explained very simply up to this point, so I'll give it a go.
You are not going to be attempting to rebuild the filesystem or in any way make it functional. This is a key point to understand; the filesystem is done. Permanently broken. This is also the reason people keep saying don't put the image on another encrypted drive; you're going to need to access it using software designed to rescue files from broken filesystems. Some of the open-source stuff may run on OpenBSD. Or it may not. The commercial offerings absolutely will not. The only operating system that supports OpenBSD's full disk encryption _is_ OpenBSD, so by putting this on an encrypted disk, you immediately rule out access from a different OS. You'll also likely be spending a lot of time with a hex editor to recover anything specific that isn't recognized by heuristics. An option that hasn't been brought up yet is to take the _unencrypted_ disk image to a specialist in drive recovery who will already have the tools, resources, and knowledge to achieve as much as is possible at this point. As for using rsync and copying files/directories, if there is a separate untouched partition, or a partition that is mounted and possibly still has things cached, copy all you can using rsync while it's still mounted.
